The present invention relates to an apparatus for measuring the autocorrelation waveform of the intensity of input light. More particularly, the present invention relates to a novel apparatus that is capable of measuring the autocorrelation waveform of the intensity of input light without employing the conventional two-photon fluorescence method or second-harmonic generation method.
Lasers such as mode-locked dye lasers, solid-state lasers and semiconductor lasers emit light pulses of the picosecond order. Such short pulses cannot be measured by a conventional method wherein the light to be measured is received by a variety of photoelectric detectors and converted into an electrical pulse signal, and wherein the electrical pulse signal is supplied to a variety of oscilloscopes for imaging on a CRT screen. To overcome this difficulty, nonlinear correlating techniques have been developed for indirectly determining the pulse width and pulse interval on the basis of the measurement of the second-order autocorrelation waveform of the intensity of input light.
Nonlinear correlating techniques include the two-photon fluoresence (TPF) method which relies upon absorption of two photons, and the second-harmonic generation (SHG) method which employs a nonlinear crystal. In both methods, the light to be measured is divided into two beams with a beam splitter, one light beam is allowed to travel over a longer distance such that it is delayed by time .tau., the two light beams are then superimposed and launched into a material that exhibits nonlinear optical effects, and an output signal from this material is examined. Since a strong output signal is obtained when two light pulses are actually superposed, one can determine the pulse width and pulse interval by measuring the intensity of the output signal as a function of delay time .tau..
FIG. 21 is a schematic drawing of the most common two-photon fluorescence system for measuring the pulse width of a strong single pulse such as the one emitted from a mode-locked pulse solid-state laser or a pulse dye laser. The laser pulse to be measured is divided into two beams of equal intensity with a 50% beam splitter 10, the two beams are reflected from mirrors 12 and 14 in such a way that they travel in opposite directions to be launched into a fluorescent liquid cell 16 which is filled with a highly fluorescent dye solution that has an energy absorption band corresponding to twice the frequency of the light to be measured and generates two-photon fluorescence light. In the cell 16, the two beams travel in opposite directions and a coordinate along the beam path is proportional to the delay time .tau.. Since strong absorption of two photons occurs in a position in the cell 16 where the two light pulses are actually superposed, the autocorrelation waveform of the laser pulse to be measured can be obtained by picking up resulting fluorescent light with a camera 18.
The two-photon fluorescence method described above has the advantage of being convenient but it is not capable of yielding as precise data as that attained by the second-harmonic generation method.
FIG. 22 is a schematic drawing showing the principle of the second-harmonic generation method for measuring the pulse width of a series of comparatively weak but very rapidly repeating pulses such as those emitted from mode-locked CW (continuous wave) dye lasers and semiconductor lasers. The laser pulse to be measured is divided into two beams with a beam splitter 20. One beam passes through a polarizer 22 to be launched into a fixed mirror 26, and the other beam passes through another polarizer 24 to be launched into a moving mirror 28 that optically produces a delay time .tau.. The polarizer 22 has a plane of polarization which is perpendicular to that of polarizer 24. The two beams reflected from the mirrors 6 and 28 are superposed by the beam splitter 20. The resulting optical signal passes through a nonlinear optical crystal (SHG crystal) 30 which generates a second harmonic wave, then through an interference filter 32 which selectively extracts the second harmonic wave, and is launched into an SHG light detector 34. The SHG crystal 30 will generate a second harmonic wave only when the two pulses are actually superposed. Consequently, by measuring the intensity of the generated second harmonic wave with the SHG detector 34, the autocorrelation waveform of the laser pulse of interest can be obtained.
An improved background-free version of the secondharmonic generation method has been recently developed; in this modification, a second harmonic wave is generated by launching two beams into the SHG crystal 30 at angles symmetrical with respect to the optical axis and an effective signal component is selectively picked up in the axial direction.
The second-harmonic generation method described above is capable of more precise measurements than the two-photon fluorescence method, but it suffers from the disadvantage that expensive materials such as KDP (Potassium Dihydrogen Phosphate), LiIO.sub.3, and ADP (Ammoniun Dihydrogen Phosphate) have to be used as the SHG crystal 30. In particular, some of these SHG materials including KDP are deliquescent and difficult to handle. In addition, the conditions for generating a second harmonic wave are very strict and in order to obtain it, phase matching conditions must be satisfied by properly controlling the temperature of the SHG crystal or the incident angle of light. This involves very complicated and cumbersome adjustments. Furthermore, the efficiency of second harmonic wave generation is generally low, e.g., only about several percent although the exact value depends on specific conditions. The interference filter 32, a pinhole plate, or some other device must be employed to separate the generated second harmonic wave from the fundamental wave. However, combined with the low SHG efficiency, it is very difficult to detect the generated second harmonic wave by such devices.